This invention relates to light guides, and more particularly relates to rigid light guides exhibiting low or no light loss while substantially preserving etendue of the guided light beam, regardless of changes in direction of the guided light beam or intersection of the guided light beam with other beams.
Rigid light guides such as solid or hollow light pipes, are an attractive, low-cost means for light collection, manipulation and transportation. Such light guides provide such functions in a much more compact volume than is possible with conventional optics. Unfortunately, with present light guide technology, bends or folds in the guides cause severe loss of light and increased etendue (cone angle), as illustrated in FIGS. 1A through 1D.
FIG. 1A shows schematically a straight light guide 10 and a guided light beam represented by outer ray traces 1 and 2. Note that the beam maintains the same cone angle as it travels along the guide, as indicated by the invariant angle α formed at the intersections of the traces. When a bend 11 is introduced into the guide 10, as shown in FIG. 1B, the cone angle is increased, as indicated by comparing angle β formed by the intersection of outer ray 3 and central ray 4 prior to encountering the bend, and angle γ formed by the intersection of these rays after the bend.
The increased cone angle of the guided beam means that fixed aperture collection devices may not be able to collect all of the light from the beam as it exits the light guide.
FIG. 1C shows another way in which light may be lost. When a second bend 12 is added to the light guide 10, some of the light, represented by outer ray traces 5 and 6, is reflected backward, as illustrated by ray 6, which reflects from an area just beyond the second bend to travel back along the bend 12 in the reverse direction, as ray 7.
FIG. 1D shows that a smooth bend 13 has an effect similar to that of the sharp bend 12 in FIG. 1B, ie, the cone angle of the guided beam is increased, as illustrated by the angle δ between rays 8 and 9 after the bend, compared to their parallel path (0 angle) before the bend.
Attempts to couple light guide sections with mirrors also leads to light loss, as shown in FIGS. 2A through 2C. In FIG. 2A, for example, the cone angle φ is such that the guided beam (indicated by outer rays 15 and 16) after exiting through exit aperture 17 of light guide 18 and being reflected by mirror 19, has an etendue too large for collection at entrance aperture 20 of light guide 21, resulting in significant coupling loss. Moving mirror 19 as close as possible so that it actually touches the edges of exit aperture 17 of light guide 18 and entrance aperture 20 of light guide 21, as illustrated in FIG. 2B, minimizes but does not eliminate the coupling loss. FIG. 2C shows the virtual image of the guided beam 22 after exiting from light guide 23 and reflection and before entry into light guide 24, illustrating the dependence of coupling loss on the cone angle θ.
FIG. 2D illustrates that coupling loss can be reduced or eliminated by inserting relay lenses 25 and 26 into the path of the guided beam before and after reflection by mirror 19. These relay lenses 25 and 26 limit the extent of the guided beam so that it fits within the entrance aperture 20 of light guide 21, avoiding coupling losses. However, such relay optics are expensive and prevent the desired compact arrangement.
Alternatives such as fiber optic bundles are also expensive. Moreover, fiber optic bundles suffer significant insertion loss because of a relatively low packing density.